Some antibiotics apparently have some antiviral properties. And antivirals do bring down antibody titers of viruses (in me). So, I can see their reasoning for wanting people off antibiotics and antivirals for 3 weeks.
In my case, I would be testing out of interest. I know I have problems with HHV-6, EBV, HSV1 and Coxsackie B. Also probably Cpn and possibly Lyme. I am currently on antivirals and feeling the best I have in 7 years. So, there is no way in the world I'm going to stop them for 3 weeks just to take a test. If I wasn't making progress I'd consider it. But, I am making progress. And since we don't know the role of XMRV in CFS and there are no established treatments, it doesn't seem wise for me to test for it at this time.
Best, Timaca